`_hb_pg_job_queue` creation date column is called `createdat`

[adam-fowler]

This is a typo we can fix, perhaps call it created_on in a similar manner to delayed_until
For consistency perhaps _hb_pg_jobs.lastmodified should include an underbar _hb_pg_jobs.last_modified

[thoven87]

I changed the schema and rename these columns in #20